J.G. Terry,
Mine (manual) came with a calibration sheet showing how far off it was at several settings. Not very. Over time, the spring that adjusts the torque setting can take a set (always store the wrench at its lowest setting to minimize this effect) and you have to calibrate it to correct that.
It is not worth buying your own torque measuring instrument unless you intend to get into the business, but if you have a large maintenance shop or engine rebuilding shop in your area they may well have one they would let you use. Otherwise, most big cities will have calibration lab services available, though they may charge you more than the wrench is worth to do the job.
The simplest way is to check calibration of the unit yourself and list and live with any error you find, allowing for it by setting the wrench to correct the error. With a wrench whose handle isn't at 90° to the work, this takes a little ingenuity. There are a number of examples of how to
check calibration of a right angle torque wrench on line.
I haven't checked mine at this point, but they say good practice is to check once a year, so I probably should. The way I would proceed is to drill a hole a little smaller than the handle through a 2×4, then split it on my bandsaw. I would use that to grip the wrench horizontally in my vice. I would take about a 21" length of flat steel stock, find the center of gravity on it and drill a hole there and square it with files for a square drive adapter on the wrench. 10" from each side of the hole I would drill holes for S hooks to hang a weight from. I would put one on each side to maintain balance. I would put the square drive adapter in the wrench and the flat stock on the wrench, then adjust the wrench position so the flat stock was horizontal. Since the calibration is in inch-lbs, with 10" of space from the wrench center, 1 lb of total weight hanging from one hook will produce 10 in-lbs of torque. Set the wrench on 10 in-lb. Hang a light plastic container that will hold a pound of water off the hook on the side that will turn the wrench clockwise looking at the flat or counterclockwise looking from the handle end of the wrench. There is no need to tare or counterbalance the weight of this container because its weight will be counted as taking part in the calibration. Now slowly fill the container with water until you just hear the wrench click. Remove the container and get its weight in pounds on an accurate scale. Multiply by 10 to get in-lbs of torque.
Repeat at the 20 in-lb setting, etc. Obviously, you will need a bigger water container for the 5 lbs needed to calibrate 50 in-lbs than you did for 10 in-lbs.
